The use of implantable prosthetic vascular grafts made of expanded, porous PTFE is well known in the art. Such vascular grafts are used to replace or bypass occluded or damaged natural blood vessels and are often implanted just below the skin to provide blood access for long term hemodialysis. Prosthetic vascular grafts, and methods of implanting the same, are generally described in Bennion et al., "Hemodialysis and Vascular Access", Vascular Surgery, pp. 625-662, 1983. Methods of forming expanded, porous PTFE tubing are well known in the art. For example, U.S. Pat. No. 4,187,390 issued to Gore discloses one such process of extruding, expanding, and then heating PTFE to produce porous, expanded PTFE structures, and the specification of such patent is hereby incorporated by reference.
In a typical technique, PTFE is provided in the form of a "coagulated dispersion" which is then extruded to a desired shape and then sintered, i.e., heated somewhat above its melting point for a time sufficient for it to coalesce into an essentially impermeable material, to produce the resulting product. A thorough discussion of PTFE coagulated dispersion powder properties and processing techniques is set forth in the technical data bulletin entitled "FLUON.RTM. PTFE COAGULATED DISPERSION POWDERS FOR PASTE EXTRUSION--SUMMARY OF PROPERTIES", pages 1-4, and in "The Processing of PTFE Coagulated Dispersion Powders", pages 1-36; both references, the contents of which are also entirely incorporated herein by reference, are available from ICI Advanced Materials, Exton, Pa 19341.
As discussed in the references incorporated herein, PTFE coagulated dispersion ("CD") polymers are typically supplied as a fine, free flowing powder. These dispersions are manufactured by coagulating an aqueous dispersion of PTFE. In order to extrude such dispersions, the coagulated dispersion polymers are blended with an "extrusion aid" or lubricant. A typical lubricant is a hydrocarbon having a desired vaporization temperature; examples are petroleum ether, naphtha, and low odor paraffin solvents. A number of such solvents, and the reasons for selecting a particular solvent, are also discussed in the references incorporated above.
The dry-appearing mix of dispersion and lubricant is them lightly pressed into a billet or preform. Such preforms are stiff and brittle and similar in consistency to soft wax candles. The preform billet is them forced through the simple die of a constant rate ram extruder. The tubular extrudate typically passes into a drying oven, the temperature of which is sufficient to vaporize the lubricant.
PTFE can be stretched to many times its original dimensions. Prior to expansion, the PTFE tubular extrudate, or "tube", is typically heated to approximately within the range of 225-300 degrees Centigrade. Upon reaching such temperature, the tube is expanded by stretching to a desired dimension. Following expansion, the PTFE tube is heated to a higher sintering temperature of approximately 375 degrees Centigrade or higher to lock the crystalline structure to its expanded dimensions. During this sintering step, the expanded PTFE tube must be held to its expanded dimensions, or the tube will contract partially back toward its pre-expanded shape.
Expanded, porous PTFE material offers a number of advantages when used as a prosthetic vascular graft. PTFE is highly biocompatible, has excellent mechanical and handling characteristics, does not require preclotting with the patient's blood, heals relatively quickly following implantation, and is thromboresistant. In general, large pore size PTFE grafts enhance vascular graft patency because grafts with large interstitial spaces improve healing by increased tissue ingrowth. However, large porosity grafts have an increased tendency to bleed and cause seroma after the graft is implanted. Therefore, a balance must be struck between a large enough pore size for good tissue ingrowth, and a small enough pore size to prevent bleeding through the graft.
U.S. Pat. No. 4,082,893 to Okita describes a PTFE tube that is processed to produce a variation in the microporous structure as between the inner surface of the tubing and the outer surface of the tubing. For example, the tubing may have pores with a 10 micron diameter at the inner surface, and pores with a 3 micron diameter at the outer surface. Variation in the microporous structure as between the inner and outer surfaces is induced by heating the outer surface of the tube to a greater degree than the inner surface of the tube, as by heating the tube externally while passing cooling air through the inner cavity of the tube.
U.S. Pat. No. 4,208,745 to Okita discloses a PTFE vascular graft wherein the fibrous structure at the inside surface of the tubing is finer than the fibrous structure at the outside surface of the tubing; for example, the tubing may have a pore size on the outside surface of at least 3 microns, and a pore size os 1 to 5 microns on the inside surface. As in the '893 Okita patent, the variation in porosity between the inner and outer surfaces is created by heating the outer surface to the sintering temperature while maintaining the inner surface at a lower temperature.
U.S. Pat. No. 4,816,339 to Tu et al is directed to a multi-layered PTFE/elastomer composite graft wherein the various layers may have different pore sizes. The different porosities within the multiple layers are caused by radial expansion of such layers. The first and second PTFE layers are preformed and extruded together to form separate layers. The resulting extrudate is expanded biaxially or uniaxially. The resulting structure is then sintered. In the preforming stage, a concentric tube is inserted inside the pre-former to divide the pre-former into two concentric spaces. The inner space is loaded with pure PTFE, while the outer space is loaded with a PTFE/elastomer mixture. Following extrusion, the extrudate is expanded and sintered. The inner layer internodal distance is described as being about 20 to 30 microns, in comparison to the second layer internodal distance, which is described as being in the range of between 30 to 500 microns.
U.S. Pat. No. 4,822,361 to Okita et al. discloses a PTFE tubular prosthesis formed from a single PTFE tube rather than from two concentric tubes. The prosthesis has a greater average fibril length on its outer surface than on its inner surface, and the fibril length is described as varying continuously across the wall of the tube. The outer surface of the tube has a larger average pore size than the inner surface of the tube; the outer surface pore size is apparently in the range of 1 to 100 microns, while the inner surface pore size is apparently in the range of from 0.1 to 1 microns. The variance in porosity from the inner surface to the outer surface is evidently accomplished by radial inflation of the tube.
Accordingly, it is an object of the present invention to provide a dual porosity PTFE tube or graft and method for making same, which tube or graft includes an inner surface of expanded PTFE having a porosity in the range of about 10-40 .mu., to reduce blood leakage, and an outer surface having a porosity in the range of about 60-155 .mu., to enhance tissue ingrowth.
It is another object of the present invention to provide a dual porosity PTFE tube or graft and method for making same, which tube or graft includes an inner surface of expanded PTFE having a porosity in the range of about 60-155 .mu., and an outer surface having a porosity in the range of about 10-40 .mu., to accelerate healing of an implanted vascular graft.
These and other objects of the present invention will become more apparent to those skilled in the art as the description thereof proceeds.